Electroacoustic Component

ABSTRACT

An electroacoustic component that includes a substrate made of monocrystalline LiNbO 3  is disclosed. In the component, a first Euler angle λ of the monocrystalline LiNbO 3  is: λ≈0°, a second Euler angle μ of the monocrystalline LiNbO 3  is: −74°≦μ≦−52° or 23°≦μ≦36°, and a third Euler angle θ of the monocrystalline LiNbO 3  is: θ≈0°.

Crystal sections of LiNbO₃ single crystals are known from the documentDE 196 41 662 B4, among others.

The problem of the present invention is to specify a component operatingwith acoustic waves, which, in addition to a low temperature coefficientof frequency (e.g., resonant frequency or center frequency), has lowerlosses due to undesired wave emission.

A component operating with acoustic waves can comprise, in particular, aresonator with a transducer in which the acoustic wave can be excited.As a rule, a transducer has an electrode grating. The electrodes aremetal strips arranged perpendicular to the wave propagation direction.The transducer can be arranged between two acoustic reflectors, whichare suitable for localizing the acoustic wave in the active region ofthe transducer. The component has a passband and at least one stopband.

The specified component can be suitable for exciting surface acousticwaves. The specified component can also be suitable for exciting guidedbulk acoustic waves—or GBAW.

A crystal section can be specified by three Euler angles. The Eulerangles will be explained below with reference to FIG. 1. The first Eulerangle is designated as λ below, the second as μ, and the third as θ.

Depending on the crystal section, a shear wave or a Rayleigh wave can beexcited in the monocrystalline LiNbO₃. It was found that theelectroacoustic coupling constant K² for the shear wave is essentiallyzero in the (0°, μ, 0°) crystal section at μ=30°, and at μ=−64° for theRayleigh wave in the (0°, μ, 0°) crystal section, i.e., the respectivewave is not, or only very weakly excited there. The respectivedifferently polarized wave is very strongly excited, however. Due to thecrystal symmetry, Euler angles (0°, μ, 0°), (0°, μ+180°, 0°) and (0°, μ,−180°, 0°) are equivalent with respect to their acoustic properties.

According to a first embodiment, an electroacoustic component isspecified with a substrate made from monocrystalline LiNbO₃ for thesecond Euler angle of which −74°≦μ≦−52°. For the first Euler angleλ:λ=0°. For the third Euler angle θ:θ=0.

An angle that is essentially equal to zero is understood to mean, inparticular, an angle range that lies between −5° and +5°. This appliesto the angles λ and θ. It can also be an angle range between −3° and+3°, however.

For the first Euler angle λ, it is preferred that λ=0°. For the thirdEuler angle θ, it is preferred that θ=0°. At the interface of thesubstrate, a horizontally polarized shear wave can be generated, whichadvantageously represents a GBAW. The horizontal polarization means thatthe shear wave is essentially polarized in a lateral plane. The anglerange in this case is particularly advantageously 67°≦μ≦−61°.

The shear wave primarily comprises a component that is polarized in theX, Y plane essentially perpendicular to the propagation direction X ofthe wave. A small wave component that is polarized perpendicular to thelateral plane can also be present.

The component preferably has a transducer in which a shear wave can bepropagated as the main mode. The transducer is distinguished in that itsadmittance curve has no secondary resonances below the resonantfrequency f_(R) of the main resonance. The secondary resonances haveonly a low intensity in the frequency range between f_(R) and 1.5f_(R).

According to a second preferred embodiment, an electroacoustic componentwith a substrate made from a monocrystalline LiNbO₃ is specified, forthe second Euler angle μ of which 23°≦μ≦36°. For the first Euler angleλ, it is preferred that λ≈0°. For the third Euler angle θ, it ispreferred that θ≈0°. In this case, a Rayleigh wave, which represents aGBAW in one advantageous variant, can be excited at the interface of thesubstrate. The angle range 28°≦μ≦32° is particularly advantageous inthis case.

The Rayleigh wave primarily comprises a component that is polarizedessentially in the sagittal plane (X, Z) perpendicular to the lateralplane. A small wave component that is polarized perpendicular to thepropagation direction X of the wave can also be present.

The component preferably has a transducer in which the Rayleigh wave iscapable of propagation as the main mode. The transducer is distinguishedin that its admittance curve below the resonant frequency f_(R) has nosecondary resonances at the main resonance.

Exemplary configurations of the component according to the first andsecond embodiments will be specified below.

A metal layer in which electrodes for exciting an acoustic wave ofwavelength λ_(o) are formed is arranged on the substrate. This refers tothe wavelength at a frequency that lies in the passband of thecomponent. The electrodes can form a periodic electrode grating of thetransducer, with a spacing between two oppositely charged electrodes ofhalf a wavelength as measured in the direction of wave propagation.

The metal layer preferably comprises a W layer, the thickness of whichrelative to the wavelength is at most 10%, and between 1% and 6.5% in anadvantageous variant. In particular, the W layer can have a thickness of5% relative to the wavelength.

The electrodes can also comprise Al or an Al alloy. For example, theycan comprise at least one Al layer, but also additional layers, a Culayer in particular. The overall height of the electrodes can be up to10% of the wavelength.

The component can be conceived as a component operating with guided bulkacoustic waves, and then comprises an additional substrate and anintermediate layer that is arranged on the first substrate with theelectrodes and is particularly suited as a planarization layer.

The additional substrate can be replaced by at least one cover layer.That which was stated in connection with the additional substrate alsoapplies to the cover layer. The cover layer can comprise several layersof different materials.

It is advantageous if the boundary surface of the intermediate layerfacing the additional substrate is planar or has been planarized. Thus,a first layer, which comprises the first substrate, the metal layer andthe intermediate layer, can be joined to the second substrate by meansof direct wafer bonding.

The metal layer is arranged between the substrate and the intermediatelayer. The intermediate layer is arranged between the metal layer andthe additional substrate. The propagation speed of the wave ispreferably larger in the substrate and in the additional substrate thanin the intermediate layer.

The proportion of metallized surface area on the surface of the firstsubstrate in the acoustically active region of the component ispreferably between 0.3 and 0.7, but is not limited to this range.

The substrate and the additional substrate each preferably have athickness of at least 7.5λ₀. The additional substrate can be made of Si,particularly of Si with the crystal section (0°, 0°, 0°).

The temperature coefficient of the frequency f of the electroacousticcomponent can be described by a Taylor series:

df/f=T0+TCF1ΔT+TCF2(ΔT)²+ . . .

df is the temperature-induced deviation of the frequency of thecomponent at a temperature difference ΔT. This can be, for instance, thetemperature deviation at room temperature or at a specified referencetemperature. The coefficient TCF1 preceding the linear term of theseries is referred to as the linear temperature coefficient. Thecoefficient TCF2 preceding the quadratic term of this series is referredto as the quadratic temperature coefficient. The curve df/f (ΔT) isessentially a straight line at a small value of the coefficient TCF2.

The crystal section, the metallization height of the electrodes and theproportion of metallized surface area in the acoustically active regionof the component are preferably selected such that the lineartemperature coefficient TCF1 is small and preferably essentially equalto zero.

The intermediate layer is preferably made of SiO_(x) with 1.6×2.1, butcan also be selected from a different material. The thickness of theintermediate layer relative to the wavelength is between 20% and 200%.The advantage of such a relatively thick intermediate layer is thatrelatively low values for the parameter TCF1 can thereby be achieved. Inthe first embodiment, for example, TCF1=−33 ppm/K for the intermediatelayer with the relative layer thickness of 25%, and TCF1=−23 ppm/K forthe intermediate layer with the relative layer thickness of 150%. In thecomponent according to the second embodiment, it was determined thatTCF1=−35 ppm/K for the intermediate layer with the relative layerthickness of 25%, and TCF1=−20 ppm/K for the intermediate layer with therelative thickness of 150%.

The advantage of a relatively thick intermediate layer is also that arelatively high coefficient of reflection R of the wave at an electrodeedge can be achieved. For a short-circuited electrode grating in thecomponent according to the second embodiment, it was determined thatR=9.34% for the intermediate layer with the relative layer thickness of25%, and R=15.07% for the intermediate layer with the relative layerthickness of 150%. The proportion of metallized surface area in theactive region of a component was 0.5 in this case. Electrodes were madeof W with a relative height of 5%.

It is possible to achieve a relatively high electroacoustic couplingcoefficient K², with K²>12% for the shear wave in the componentaccording to the first embodiment, and K²>6% for the Rayleigh wave inthe component according to the second embodiment.

With regard to the coupling coefficient K in the component according tothe first embodiment, K²=15.08% was determined for the intermediatelayer with the relative layer thickness 25%, and K²=12.34% wasdetermined for the intermediate layer with the relative layer thicknessof 150%, wherein the proportion of metallized surface area in the activeregion of the component was 0.5.

In the component according to the second embodiment, K²=7.74% wasdetermined for the intermediate layer with the relative layer thickness25%, and K²=6.31% was determined for the intermediate layer with therelative layer thickness of 150%, wherein the proportion of metallizedsurface area in the active range of the component was 0.5.

The component will be explained with reference to schematic figures notdrawn to scale. These show:

FIG. 1, explanations of the Euler angles for a crystal section; and

FIG. 2, an exemplary component operating with GBAW.

The Euler angles are explained with reference to FIG. 1. The axes of thecrystal-physical coordinate system (x, y, z) are oriented along thecrystal axes (a, b, c) of an elementary cell of the single crystal. Thefirst Euler angle λ describes a rotation of the coordinate system aboutthe z-axis, see FIG. 1. The once-rotated coordinate system is designated(x′, y′, z). The second Euler angle μ describes a rotation of theonce-rotated coordinate system about the x′-axis. Then one transitionsto the (x′, y″, z) coordinate system. The third Euler angle θ describesa rotation of the twice-rotated coordinate system about the Z-axis. TheX-axis of the coordinate system (X, Y, Z) thus obtained is oriented inthe direction of propagation of the acoustic wave. The acoustic wavepropagates in the X, Y plane, also referred to as the section plane ofthe substrate. The Z-axis is the normal line to this plane.

A cutout of a component with a transducer W1, in which a guided bulkacoustic wave GBAW can be excited, is shown in FIG. 2.

The first substrate is lithium niobate with one of the specified crystalsections. A metallization layer having a transducer W1 and a contactarea KF1 connected thereto is arranged on first substrate S1. Thetransducer comprises electrodes E1, E2, wherein first electrodes E1 andsecond electrodes E2 are arranged alternately in the wave propagationdirection X. They each extend perpendicular to this direction in thelateral plane.

The structures E1, E2, KF1 of the metal layer are covered with anintermediate layer ZS which seals off the surface of first substrate 51that remains exposed. Intermediate layer ZS is at least as high as thismetal layer.

Contact area KF1 can be contacted from the outside by means of a platedthrough-hole DK through second substrate S₂ and intermediate layer ZS.This plated through-hole represents a hole whose surface is covered witha metallization. The metallization lies on top of the exposed surface ofsecond substrate S2 and forms an external contact AE.

The component is not limited to the example shown in FIG. 2.

LIST OF REFERENCE CHARACTERS

-   AE External electrode-   DK Plated through-hole-   E1, E2 Electrodes-   KF1 Contact area-   S1 First substrate-   S2 Second substrate-   ZS Intermediate layer-   X Wave propagation direction-   Y, Z Spatial directions-   x, x′, y, y′, y″, z Spatial directions-   λ First Euler angle-   μ Second Euler angle-   θ Third Euler angle

1. An electroacoustic component, comprising: a substrate comprisingmonocrystalline LiNbO₃, wherein: a first Euler angle λ of themonocrystalline LiNbO₃ is: λ≈0°, a second Euler angle μ of themonocrystalline LiNbO₃ is: −74°≦μ≦−52°, an a third Euler angle θ of themonocrystalline LiNbO₃ is: θ≈0°.
 2. The component of claim 1, whereinthe component is configured such that a horizontally polarized shearwave can be excited in the component.
 3. An electroacoustic component,comprising: a substrate comprising monocrystalline LiNbO₃, wherein: afirst Euler angle λ of the monocrystalline LiNbO₃ is: λ≈0°, a secondEuler angle μ of the monocrystalline LiNbO₃ is: 23°≦μ≦36°, and a thirdEuler angle θ of the monocrystalline LiNbO₃ is: θ≈0°.
 4. The componentof claim 3, wherein the component is configured such that a Rayleighwave can be excited in the component.
 5. The component of claim 1,further comprising with a metal layer on the substrate, the metal layercomprising electrodes for exciting an acoustic wave with a wavelengthλ_(o).
 6. The component of claim 5, wherein the metal layer comprises aW layer having a thickness relative to the wavelength of at most about10%.
 7. The component of claim 5, wherein the component is configured tooperate with guided bulk acoustic waves, and the component furthercomprises: an additional substrate; and an intermediate layer, wherein:the metal layer is between the substrate and the intermediate layer, andthe intermediate layer is between the metal layer and the additionalsubstrate, a propagation speed of the bulk acoustic wave in thesubstrate and in the additional substrate is greater than a propagationspeed of the bulk acoustic wave that in the intermediate layer.
 8. Thecomponent of claim 5, wherein the component is configured to operatewith guided bulk acoustic waves, and the component further comprises: acover layer; and an intermediate layer, wherein: the metal layer isbetween the substrate and the intermediate layer, the intermediate layeris between the metal layer and the cover layer, and a propagation speedof the bulk acoustic wave in the substrate and the cover layer isgreater than a propagation speed of the bulk acoustic wave in theintermediate layer.
 9. The component of claim 7, wherein theintermediate layer comprises SiO_(x) with 1.6≦x≦2.1.
 10. The componentof claim 9, wherein the intermediate layer comprises SiO_(x) with1.95≦x≦2.05.
 11. The component of claim 7, wherein the thickness of theintermediate layer is between 20% and 200% relative to the wavelengthλ₀.
 12. The component of claim 7, wherein the additional substratecomprises Si.
 13. The component of claim 7, wherein the substrate andthe additional substrate each has a thickness of at least 10λ₀.
 14. Thecomponent of claim 1, wherein for the second Euler angle μ: −67°≦μ−61°.15. The component of claim 3, wherein for the second Euler angle μ:28°≦μ32°.
 16. The component of claim 3, further comprising a metal layeron the substrate, the metal layer comprising electrodes for exciting anacoustic wave with a wavelength λ_(o).
 17. The component of claim 16,wherein the metal layer comprises a W layer, wherein a thickness of theW layer relative to the wavelength is at most about 10%.
 18. Thecomponent of claim 16, wherein the component is configured to operatewith guided bulk acoustic waves, and the component further comprises: anadditional substrate; and an intermediate layer, wherein: the metallayer is between the substrate and the intermediate layer, theintermediate layer is between the metal layer and the additionalsubstrate, and a propagation speed of the bulk acoustic wave in thesubstrate and in the additional substrate is greater than a propagationspeed of the bulk acoustic wave in the intermediate layer.
 19. Thecomponent of claim 16, wherein the component is configured to operatewith guided bulk acoustic waves, and the component further comprises: acover layer; and an intermediate layer, wherein: the metal layer isbetween the substrate and the intermediate layer, the intermediate layeris between the metal layer and the cover layer, and a propagation speedof the bulk acoustic wave in the substrate and the cover layer isgreater than a propagation speed of the bulk acoustic wave in theintermediate layer.
 20. The component of claim 18, wherein theintermediate layer comprises SiO_(x) with 1.6≦x≦2.1.
 21. The componentof claim 20, wherein the intermediate layer comprises SiO_(x) with1.95≦x≦2.05.
 22. The component of claim 18, wherein the thickness of theintermediate layer is between 20% and 200% relative to the wavelengthλ_(o).
 23. The component of claim 18, wherein the additional substratecomprises Si.
 24. The component of claim 18, wherein the substrate andthe additional substrate each has a thickness of at least 10λ₀.